Programming paradigms

Imperative

Imperative programming in D is almost identical to that in C. Functions, data, statements, declarations and expressions work just as they do in C, and the C runtime library may be accessed directly. On the other hand, some notable differences between D and C in the area of imperative programming include D's foreach loop construct, which allows looping over a collection, and nested functions, which are functions that are declared inside of another and may access the enclosing function's local variables.

Object-oriented

Object-oriented programming in D is based on a single inheritance hierarchy, with all classes derived from class Object. D does not support multiple inheritance; instead, it uses Java-style interfaces, which are comparable to C++'s pure abstract classes, and mixins, which separates common functionality from the inheritance hierarchy. D also allows the defining of static and final (non-virtual) methods in interfaces.

Metaprogramming

Metaprogramming is supported by a combination of templates, compile time function execution, tuples, and string mixins. The following examples demonstrate some of D's compile-time features.

Templates in D can be written in a more imperative style compared to the C++ functional style for templates. This is a regular function that calculates the factorial of a number:

ulongfactorial(ulongn){if(n<2)return1;elsereturnn*factorial(n-1);}

Here, the use of static if, D's compile-time conditional construct, is demonstrated to construct a template that performs the same calculation using code that is similar to that of the function above:

In the following two examples, the template and function defined above are used to compute factorials. The types of constants need not be specified explicitly as the compiler infers their types from the right-hand sides of assignments:

enumfact_7=Factorial!(7);

This is an example of compile time function execution. Ordinary functions may be used in constant, compile-time expressions provided they meet certain criteria:

enumfact_9=factorial(9);

The std.string.format function performs printf-like data formatting (also at compile-time, through CTFE), and the "msg" pragma displays the result at compile time:

String mixins, combined with compile-time function execution, allow generating D code using string operations at compile time. This can be used to parse domain-specific languages to D code, which will be compiled as part of the program:

There are two built-in types for function literals, function, which is simply a pointer to a stack-allocated function, and delegate, which also includes a pointer to the surrounding environment. Type inference may be used with an anonymous function, in which case the compiler creates a delegate unless it can prove that an environment pointer is not necessary. Likewise, to implement a closure, the compiler places enclosed local variables on the heap only if necessary (for example, if a closure is returned by another function, and exits that function's scope). When using type inference, the compiler will also add attributes such as pure and nothrow to a function's type, if it can prove that they apply.

Other functional features such as currying and common higher-order functions such as map, filter, and reduce are available through the standard library modules std.functional and std.algorithm.

Parallel

importstd.stdio:writeln;importstd.range:iota;importstd.parallelism:parallel;voidmain(){foreach(i;iota(11).parallel){// The body of the foreach loop is executed in parallel for each iwriteln("processing ",i);}}

Concurrent

importstd.stdio,std.concurrency,std.variant;voidfoo(){boolcont=true;while(cont){receive(// delegates are used to match the message type(intmsg)=>writeln("int received: ",msg),(Tidsender){cont=false;sender.send(-1);},(Variantv)=>writeln("huh?")// Variant matches any type);}}voidmain(){autotid=spawn(&foo);// spawn a new thread running foo()foreach(i;0..10)tid.send(i);// send some integerstid.send(1.0f);// send a floattid.send("hello");// send a stringtid.send(thisTid);// send a struct (Tid)receive((intx)=>writeln("Main thread received message: ",x));}

Memory management

Memory is usually managed with garbage collection, but specific objects may be finalized immediately when they go out of scope. Explicit memory management is possible using the overloaded operatorsnew and delete, and by simply calling C's malloc and free directly. Garbage collection can be controlled: programmers may add and exclude memory ranges from being observed by the collector, can disable and enable the collector and force either a generational or full collection cycle.[12] The manual gives many examples of how to implement different highly optimized memory management schemes for when garbage collection is inadequate in a program.[13]

SafeD

SafeD[14]
is the name given to the subset of D that can be guaranteed to be memory safe (no writes to memory that were not allocated or that have already been recycled). Functions marked @safe are checked at compile time to ensure that they do not use any features that could result in corruption of memory, such as pointer arithmetic and unchecked casts, and any other functions called must also be marked as @safe or @trusted. Functions can be marked @trusted for the cases where the compiler cannot distinguish between safe use of a feature that is disabled in SafeD and a potential case of memory corruption.[15]

Interaction with other systems

C's application binary interface (ABI) is supported, as well as all of C's fundamental and derived types, enabling direct access to existing C code and libraries. D bindings are available for many popular C libraries. Additionally, C's standard library is a part of standard D.

Because C++ does not have a single standard ABI, D can only fully access C++ code that is written to the C ABI. The D parser understands an extern (C++) calling convention for limited linking to C++ objects.

History

Walter Bright decided to start working on a new language in 1999. D was first released in December 2001,[1] and reached version 1.0 in January 2007.[16] The first version of the language (D1) concentrated on the imperative, object oriented and metaprogramming paradigms,[17] similar to C++.

Dissatisfied with Phobos, D's official runtime and standard library, members of the D community created an alternative runtime and standard library named Tango. The first public Tango announcement came within days of D 1.0's release.[18] Tango adopted a different programming style, embracing OOP and high modularity. Being a community-led project, Tango was more open to contributions, which allowed it to progress faster than the official standard library. At that time, Tango and Phobos were incompatible due to different runtime support APIs (the garbage collector, threading support, etc.). This made it impossible to use both libraries in the same project. The existence of two libraries, both widely in use, has led to significant dispute due to some packages using Phobos and others using Tango.[19]

In June 2007, the first version of D2 was released.[2] The beginning of D2's development signalled the stabilization of D1; the first version of the language has been placed in maintenance, only receiving corrections and implementation bugfixes. D2 was to introduce breaking changes to the language, beginning with its first experimental const system. D2 later added numerous other language features, such as closures, purity, and support for the functional and concurrent programming paradigms. D2 also solved standard library problems by separating the runtime from the standard library. The completion of a D2 Tango port was announced in February 2012.[20]

The release of Andrei Alexandrescu's book The D Programming Language on 12 June 2010 marked the stabilization of D2, which today is commonly referred to as just "D".

In January 2011, D development moved from a bugtracker / patch-submission basis to GitHub. This has led to a significant increase in contributions to the compiler, runtime and standard library.[21]

In December 2011, Andrei Alexandrescu announced that D1, the first version of the language, would be discontinued on 31 December 2012.[22] The final D1 release, D v1.076, was on 31 December 2012.[23]

SDC – The Stupid D Compiler uses a custom front-end and LLVM as its compiler back-end. It is written in D and uses a scheduler to handle symbol resolution in order to elegantly handle the compile-time features of D. This compiler currently supports a limited subset of the language.[34][35]

A bundle is available for TextMate, and the Code::Blocks IDE includes partial support for the language. However, standard IDE features such as code completion or refactoring are not yet available, though they do work partially in Code::Blocks (due to D's similarity to C).

A plugin for Xcode 3 is available, D for Xcode, to enable D-based projects and development.[43]

D applications can be debugged using any C/C++ debugger, like GDB or WinDbg, although support for various D-specific language features is extremely limited. On Windows, D programs can be debugged using Ddbg, or Microsoft debugging tools (WinDBG and Visual Studio), after having converted the debug information using cv2pdb. The ZeroBUGS debugger for Linux has experimental support for the D language. Ddbg can be used with various IDEs or from the command line; ZeroBUGS has its own graphical user interface (GUI).

Examples

Example 1

This example program prints its command line arguments. The main function is the entry point of a D program, and args is an array of strings representing the command line arguments. A string in D is an array of characters, represented by char[] in D1, or immutable(char)[] in D2.

The foreach statement can iterate over any collection. In this case, it is producing a sequence of indexes (i) and values (arg) from the array args. The index i and the value arg have their types inferred from the type of the array args.

Example 2

The following shows several D capabilities and D design trade-offs in a very short program. It iterates over the lines of a text file named words.txt, which contains a different word on each line, and prints all the words that are anagrams of other words.

signs2words is a built-in associative array that maps dstring (32-bit / char) keys to arrays of dstrings. It is similar to defaultdict(list) in Python.

lines(File()) yields lines lazily, with the newline. It has to then be copied with idup to obtain a string to be used for the associative array values (the idup property of arrays returns an immutable duplicate of the array, which is required since the dstring type is actually immutable(dchar)[]). Built-in associative arrays require immutable keys.

The ~= operator appends a new dstring to the values of the associate dynamic array.

toLower, join and chomp are string functions that D allows the use of with a method syntax. The name of such functions is often very similar to Python string methods. The toLower converts a string to lower case, join(" ") joins an array of strings into a single string using a single space as separator, and chomp removes a newline from the end of the string if one is present.

The sort is an std.algorithm function that sorts the array in place, creating a unique signature for words that are anagrams of each other. The release() method on the return value of sort() is handy to keep the code as a single expression.

The second foreach iterates on the values of the associative array, it's able to infer the type of words.

key is assigned to an immutable variable, its type is inferred.

UTF-32dchar[] is used instead of normal UTF-8char[] otherwise sort() refuses to sort it. There are more efficient ways to write this program that use just UTF-8.